Shooting off - the act of masturbation. Originally meaning to leave or go somewhere (e.g. men may go shooting off to outer space..., I've got to shoot off now, will talk later though), but has come to mean wanking.

It was really awkward when Garr walked into the room and I was shooting off.

Person 1: Any plans for this evening?
Person 2: Not really. Might go shooting off later.
Person 1: Ok. Remember to clean it up afterwards :)